The invention relates to a circuit configuration for low-power reference voltage generation.
A multiplicity of circuits for reference voltage generation are known, all of which, however, have a high intrinsic power consumption which reduces the operating period of, particularly, battery-operated applications and devices. In monolithic circuits, band gap references are frequently used which generate a constant reference voltage from a fluctuating voltage supply. However, band gap references themselves need a supply current of over 10 xcexcA. Voltage regulators, too, usually have a band gap reference in order to generate a regulated supply voltage from a fluctuating voltage.
A circuit configuration for voltage generation is known from Published, Non-Prosecuted German Patent Application DE 31 05 198 A1. The circuit configuration described there contains a programmable voltage source which has an operational amplifier connected as an amplifier with a programmable voltage divider connected in its feedback path. The programmable voltage divider is used for setting the gain factor of the amplifier. At the output of the amplifier, an output voltage can be picked up. The circuit configuration also contains a calibration device which contains a potentiometer, a rectifier, a window discriminator and a comparator. At the input of the potentiometer, an alternating voltage is superimposed on an input voltage. The voltage present at the output of the potentiometer is rectified by the rectifier and supplied both to the window discriminator and to the comparator. The window discriminator and the comparator determine by comparisons with predetermined reference voltages whether the output tap of the potentiometer is located in the vicinity of one end of the potentiometer and towards which end it is moving. At the output end, the window discriminator and the comparator generate signals by which they feed a control device in the form of a counter. Depending on the signals fed in, the control device sets the gain factor of the programmable voltage source via a multiplexer.
A further circuit configuration for voltage generation is known from the Published, Non-Prosecuted French Patent Application FR 2 749 457 A3. This circuit configuration is used for converting a digital signal into an analog output voltage. The digital signal is input into decoders, one of which converts the more significant bits and the other one of which converts the less significant bits into analog signals. The analog signals are used for programming a voltage source. The programmable voltage source contains an operational amplifier, at the output of which the output voltage can be picked up. The non-inverting input of the operational amplifier is fed with a voltage that is generated by a voltage divider that contains a plurality of resistors and a plurality of controllable switches. The switches are controlled by the more significant bits of the digital signal. The inverting input of the operational amplifier is connected to the output of the operational amplifier via a feedback branch. A resistor is connected into the feedback branch. In addition, a current is fed into the feedback branch that is generated by a plurality of other controllable switches, transistors and other resistors. The further switches are controlled by the less significant bits of the digital signal. The magnitude of the output voltage is essentially determined by the current feeding the feedback branch and the voltage present at the non-inverting input of the operational amplifier. Furthermore, the output voltage feeds a circuit that supplies the output voltage as input quantity to a function. At the output end, the circuit generates a voltage that is applied to control terminals of the other resistors.
Overall, the circuit configuration described above shows that the more significant bits are used for coarse setting of the output voltage and the less significant bits are used for fine setting the output voltage.
It is accordingly an object of the invention to provide a circuit configuration for low-power reference voltage generation which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which only needs a low supply current.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit configuration for low-power reference voltage generation. The circuit configuration containing a programmable voltage source for generating an output voltage, a calibration device for deriving at least one signal in dependence on a comparison of the output voltage with a reference voltage at predetermined times, and a control device connected to and receiving the signal from the calibration device. The control device is connected to and programs the programmable voltage source so that the output voltage substantially corresponds to the reference voltage.
The invention relates to a circuit configuration for low-power reference voltage generation, in which a programmable voltage source generates an output voltage. The output voltage is compared with a reference voltage at predetermined times. Depending on the comparison, at least one signal, which is supplied to a control device, is derived by a calibration device. The control device programs the voltage source in such a manner that the output voltage corresponds to the reference voltage as closely as possible.
In this configuration, the actual regulated output voltage is generated by the programmable voltage source. A reference voltage source with a high current demand is only needed at the predetermined times when the output voltage is compared with the reference voltage. The programmable voltage source is used for simulating a reference voltage source, as it were, with the advantage that the programmable voltage source can be configured as a very low-power device. An advantage of the method thus relates to the fact that the reference voltage is only needed at certain times and, as a result, it is not necessary to operate a reference voltage source continuously. For example, a band gap reference providing the reference voltage is only switched on at predetermined times and switched off again in-between times. The method considerably lowers the current demand particularly if the programmable voltage source is recalibrated again only at relatively large time intervals.
The programmable voltage source is preferably configured as a programmable voltage divider that is fed by a voltage source. The current consumption of the programmable voltage source can be distinctly lowered particularly by high resistance values of the voltage divider. A further advantage lies in the simple structure of the voltage divider and of the voltage source feeding the voltage divider. If high resistance values are used for the voltage divider, it is not only the current consumption that is lowered but the feeding voltage source is also loaded to a lesser extent.
The voltage divider preferably has a multiplicity of series-connected resistors and individual resistors of the voltage divider can be bridged in each case by a switch. This embodiment can be advantageously constructed in a very simple circuit. As an alternative, the voltage divider can also be constructed as a parallel connection of resistors. In an integrated circuit in the currently available semiconductor and integration technologies, however, this embodiment requires a larger area.
The values of the resistors are preferably graduated in such a manner that the values of in each case series-connected resistors differ by a factor of two and each resistance value is a multiple of a predetermined resistance value. This makes it possible to achieve a finely graduated characteristic of the voltages programmable by the voltage divider. In addition, the ratios between resistors can be achieved more accurately than absolute values, especially in integrated circuit technology.
The control device programs the voltage divider preferably by closing or opening individual switches. The switches are preferably constructed as MOSFET transistors of the enrichment type. This embodiment facilitates an integration of the method with other circuits, particularly in a monolithic CMOS circuit. MOSFET transistors have been found to be successful as switches in digital technology due to their good switching characteristics and their low load path resistance and are thus also suitable for the almost resistanceless bridging of individual resistors of the voltage divider. It is especially MOSFET transistors of the enrichment type which are suitable as switches since these transistors only begin to conduct from a certain control voltage and thus reliably cut off with a control voltage of 0 V and slightly above.
The control device preferably digitally stores the programming of the voltage divider. Digital storage of the programmed setting of the voltage divider can be achieved, on the one hand, by very simply achieved, particularly in integrated circuit technology and, on the other hand, is more reliable than, for example, analog storage which is afflicted with losses and with which adequate long-term stability, for example over a number of weeks, can scarcely be achieved, particularly due to leakage currents.
The control device is constructed particularly preferably as a digital counter with an upcounting and downcounting function. Digital counters are available in a multiplicity of embodiments, can be implemented by simple construction and, in particular, can be constructed as a very low-power device, particularly in CMOS technology.
The digital counter is preferably clocked by a counting clock, the counting pulses of which correspond to the predetermined times for comparing the output voltage.
The times for comparing the output voltage with the reference voltage are preferably predetermined in dependence on fluctuations or changes in the output voltage. In the case of fluctuations with little spacing in time, a calibration must be performed correspondingly more frequently than in the case of fluctuations with a large spacing in time.
The voltage divider is especially preferably followed by a regulating element that is controlled by the voltage divider in such a manner that when the output voltage of the voltage divider drops below a predetermined voltage, the regulating element decreases the current flow. The regulating element is preferably constructed as an n-channel MOSFET transistor of the enrichment type connected as a common-source circuit or, for example, in BICMOS technology as an npn bipolar transistor connected as an emitter-follower circuit. This prevents, on the one hand, the voltage divider from being loaded by too high an output current and, on the other hand, buffer capacitors from becoming discharged via resistors in the circuit, particularly when the supply voltage drops.
Finally, a positive temperature coefficient of the regulating element is preferably compensated for by diodes connected into the voltage divider.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit configuration for low-power reference voltage generation, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.